Split offline hiking trails per country, add a picker instead of one big file

data/hiking_trails.geojson is now data/hiking_trails/<CODE>.geojson —
one file per country instead of a merge, generated per-source
(PmtilesBuilder.extract_hiking_trails), cached the same way
geofabrik_cache is (skipped once a country's own extract is already
newer than its source .osm.pbf, so growing the selection by one
country only re-extracts that one). GET /api/hiking-trails now lists
available country codes instead of a single file's status.

The Overlays panel's "Hiking trails" item becomes an expandable
per-country checklist once any offline extract exists (same shape as
Radar's own expand), replacing the plain on/off checkbox — nobody
covering a dozen countries' worth of offline map data needs all of
their hiking trails loaded and parsed at once, so each ticked country
fetches its own GeoJSON independently instead.

HikingTrails.extract_merged (server-side multi-country merge, no
longer needed now that the client picks per country) removed again.
Verified the new extract_hiking_trails + caching skip live on brick
against a real cached country PBF.

app.rb

@@ -62,7 +62,6 @@ module Maptool
 
     OFFLINE_PMTILES_PATH = File.join(DATA_DIR, "offline.pmtiles")
     TERRAIN_PMTILES_PATH = File.join(DATA_DIR, "terrain.pmtiles")
-    HIKING_TRAILS_PATH = PmtilesBuilder.hiking_trails_path(DATA_DIR)
     GPX_STORE = GpxStore.new(File.join(DATA_DIR, "gpx"))
     LIVETRACK_STORE = LiveTrackStore.new(File.join(DATA_DIR, "livetrack.json"))
 
@@ -361,24 +360,28 @@ module Maptool
     # PmtilesBuilder.extract_hiking_trails) rather than its own separate
     # build — it's derived from the exact same per-country .osm.pbf cache
     # offline.pmtiles itself already downloads, so there's nothing of its
-    # own to trigger or delete; DELETE /api/offline-tiles removes this
-    # file too. No `meta` argument here (unlike offline/terrain above) —
-    # parsing the whole GeoJSON just to report a feature count would cost
-    # real time on every page load's status check for what's a multi-MB
-    # file across several countries; size/mtime alone is enough to know
-    # whether it's there.
+    # own to trigger; DELETE /api/offline-tiles removes it too. One file
+    # per country (not one combined file) so a viewer only ever fetches
+    # the country they actually pick in the Hiking trails overlay's own
+    # checklist (public/js/hiking_trails.js), not every selected country's
+    # trails at once — `countries` lists which ones exist to build that
+    # checklist from.
     get "/api/hiking-trails" do
-      json pmtiles_file_status(HIKING_TRAILS_PATH, nil)
+      json countries: PmtilesBuilder.hiking_trails_countries(DATA_DIR)
     end
 
-    # Fetched client-side only when the Hiking trails overlay is actually
-    # toggled on (public/js/hiking_trails.js), not on page load — same
-    # "on demand" story as the online raster tiles it replaces once this
-    # is available.
-    get "/tiles/hiking_trails.geojson" do
-      halt 404 unless File.exist?(HIKING_TRAILS_PATH)
+    # Fetched client-side only for a country the viewer actually ticks in
+    # the Hiking trails checklist, not on page load. :code is constrained
+    # to exactly what hiking_trails_countries can ever return (two
+    # uppercase letters) — otherwise this would accept an arbitrary
+    # ".."-laden path segment.
+    get "/tiles/hiking_trails/:code.geojson" do
+      halt 404 unless params[:code].match?(/\A[A-Z]{2}\z/)
 
-      send_file HIKING_TRAILS_PATH, type: "application/geo+json"
+      path = PmtilesBuilder.hiking_trails_country_path(DATA_DIR, params[:code])
+      halt 404 unless File.exist?(path)
+
+      send_file path, type: "application/geo+json"
     end
 
     # Pinged once per page load (public/js/map.js) — url is a same-origin